The present invention relates generally to computed tomography imaging and, more particularly, to a method and apparatus of multi-phase cardiac CT imaging.
The narrowing or constriction of vessels carrying blood to the heart is a well-known cause of heart attacks and, gone untreated, can lead to sudden death. In such stenotic vessels, it is known that the region immediately downstream from the constriction is characterized by having rapid flow velocities and/or complex flow patterns. In general, narrowing of blood carrying vessels supplying an organ will ultimately lead to compromised function of the organ in question, at best, and organ failure, at worst. Quantitative flow data can readily aid in the diagnosis and management of patients and also help in the basic understanding of disease processes. There are many techniques available for the measurement of blood flow, including imaging based methods using radiographic imaging of contrast agents, both in projection and computed tomography (CT), ultrasound, and nuclear medicine techniques. Radiographic and nuclear medicine techniques often require the use of ionizing radiation and/or contrast agents. Some methods involve making assumptions about the flow characteristics which may not necessarily be true in vivo or require knowledge about the cross-sectional area of the vessel or the flow direction.
CT is one technique of acquiring blood flow and other cardiac data. Typically, in CT imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage, for example. Hereinafter, reference to a “subject” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam of radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to a data processing system for subsequent image reconstruction by an image reconstructor.
Cardiac reconstruction represents a significant portion of the processing required for CT cardiac imaging applications. The number of images to be reconstructed in cardiac studies can be quite large, especially if multiple phases of cardiac images are prescribed. Multiple phases are required for some cardiac applications such as left ventricular (LV) function where ejection fraction is calculated and LV wall motion is assessed. As such, it is not uncommon for 10 to 20 distinct phase locations to be required adequate temporal sampling. Likewise, multiple phases are desirable for imaging the coronary arteries as several phases may be required, frequently 3 or 6 phases, to sufficiently “freeze” the motion of the major arteries.
In multi-phase reconstruction it is generally known that the same scan data could be used for making images at the same location over a number of phases of the cardiac cycle. Known multi-phase reconstruction techniques reconstruct images for one phase at a time in what is generally referred to as a phase location driven reconstruction process. With this reconstruction technique, images for one phase are reconstructed for all imaging locations before proceeding with image reconstruction for another phase. As a result, to reconstruct all the multi-phase images, it is necessary to retrieve from memory and preprocess the same scan data a multiple number of times. These iterations decrease the performance and lengthen the time of the overall imaging process.
Therefore, it would be desirable to design an apparatus and method of multi-phase cardiac CT imaging that adaptively selects between a phase location driven reconstruction process and a more time efficient non-phase location driven reconstruction process.